1. Technical Field
This invention generally relates to a system for determining the speed of a motor vehicle. More particularly, the present invention relates to a system for calculating the speed of a motor vehicle from distance pulses utilizing acceleration.
2. Discussion
It is generally known in the automobile art that distances travelled and speeds obtained are determinable through interpretation of distance signals consisting of a series of distance pulses. Generally, a pulse generator is configured to generate a predetermined sequence of pulses, comprising a pulse train, corresponding to vehicle speed or distance travelled. The pulse train is interpreted in a controller and a value representing vehicle speed or distance travelled is calculated.
Distance sensors are commonly employed for generating such pulse trains and are usually factored to generate 8000 pulses per mile of vehicle travel or 8 pulses per distance sensor driving member revolution which roughly corresponds to wheel rotation. More recently, electronic transmissions have been substituted for mechanical transmissions. The electronic transmissions generate pulses mimicking the distance sensors by sensing vehicle speeds and generating a pulse train accordingly.
Distance sensors and electronic transmissions are adequate for fairly accurately incrementing motor vehicle odometers and for engine control operations. However, due to frequently inconsistent spacing between subsequent pulses along the pulse train, speed calculations based solely on pulse trains from distance sensors are crude, inaccurate, and unreliable. Distance sensors often generate pulses along a pulse train at different lengths from each other even though the vehicle is travelling at a constant speed. Thus, systems calculating speed with distance sensors or electronic transmissions require a controller to be implemented to improve system accuracy.
Generally, the controller roughly estimates vehicle velocity at speeds below a predetermined level, such as twenty miles per hour. At speeds in excess of the predetermined level, the spacing between pulses along the pulse train from the distance sensor or electronic transmission have become more frequent and permit more accurate speed calculations.
However, the signal generated by these systems is choppy and requires smoothing prior to driving the speedometer. A body controller is often required for smoothing the signal so that a smooth speedometer display is achieved. However, by adding smoothing or filtering, the system does not respond to changes in vehicle speed very well. Under extreme acceleration or deceleration conditions, the speedometer lags behind the true vehicle speed. Also, shifting is difficult to control when the calculated speed lags behind the true vehicle speed.
Additionally, none of the prior art systems accurately account for changes in vehicle velocity when computing vehicle speed. Acceleration or deceleration is accounted for merely by averaging a series of speed calculations together and attempting to distinguish between accelerating and decelerating states. Therefore, speed estimates overshoot and undershoot true vehicle speed.
Therefore, it is desirable to provide a system for calculating motor vehicle speed by accurately compensating for vehicle acceleration and deceleration to minimize speedometer lag. It is also desirable to provide a system for calculating motor vehicle speed which utilizes system tolerance for determining the acceleration, deceleration or steady state of the vehicle. It is further desirable to eliminate the smoothing step from the system and to provide a speed signal more quickly, smoothly and accurately than according to the prior art.